Exhasust temperature versus turbine pressure ratio based turbine control method and device

ABSTRACT

Gas turbine, software and method for controlling an operating point of the gas turbine that includes a compressor, a combustor and at least a turbine is provided. The method includes determining a turbine exhaust pressure at an exhaust of the turbine; measuring a compressor pressure discharge at the compressor; determining a turbine pressure ratio based on the turbine exhaust pressure and the compressor pressure discharge; calculating an exhaust temperature at the exhaust of the turbine as a function of the turbine pressure ratio; identifying a reference exhaust temperature curve in a plane defined by the exhaust temperature and the turbine pressure ratio; and controlling the gas turbine to maintain the operating point on the reference exhaust temperature curve.

BACKGROUND

1. Technical Field

Embodiments of the subject matter disclosed herein generally relate tomethods and systems and, more particularly, to mechanisms and techniquesfor controlling a turbine.

2. Discussion of the Background

Turbomachinery used, for example, in power plants or jet engines, arecontinuously evolving based on new discoveries and better materials. Inaddition, the manufacturers of these machines are under increasedpressure to produce or improve the machines to be “greener,” i.e., toreduce the amount of pollution produced while operating.

Thus, there is ongoing research for lowering the exhaust emissions ofturbo machineries especially considering the desire to use a wide rangeof gaseous fuels. Meeting these requirements becomes more and moredifficult, particularly when considering the wide range of operation ofthese devices. An accurate turbomachine exhaust temperature controlbecomes, under these conditions, a relevant factor in order to developsuccessful applications.

One approach for lowering the pollution produced by a turbomachine isbased on a paradigm of exhaust temperature versus compressor pressureratio. In this regard, U.S. Patent Application Publication 2008/0243352,the entire content of which is included here by reference, describesthat current control systems may execute scheduling algorithms thatadjust the fuel flow, inlet guide vanes (IGV), and other control inputsto provide safe and efficient operation of a gas turbine. Gas turbinecontrol systems may receive as inputs operating parameters and settingsthat, in conjunction with scheduling algorithms, determine turbinecontrol settings to achieve the desired operation. Measured operatingparameters may include compressor inlet pressure and temperature,compressor exit pressure and temperature, turbine exhaust temperature,and generator power output. Desired operating settings may includegenerator power output and exhaust energy. The schedules (e.g., exhausttemperature vs. compressor pressure ratio, fuel splits vs. combustionreference temperature, inlet bleed heat (IBH) vs. IGV, compressoroperating limit line vs. corrected speed and inlet guide vane, etc.) aredefined to protect the turbine against known operational boundaries(e.g., emissions, dynamics, lean-blow-out, compressor surge, compressoricing, compressor clearances, aero-mechanical, etc.) based on off-linefield tests or laboratory data. The output of the schedules thendetermines the appropriate adjustment of the control system inputs.Typical control inputs managed by a control system may include fuelflow, combustor fuel distribution (which may be referred to as “fuelsplits”), compressor inlet guide vane position, and inlet bleed heatflow.

FIG. 1, which is similar to FIG. 1 of U.S. Patent ApplicationPublication 2008/0243352, illustrates an example of a gas turbine 10having a compressor 12, a combustor 14, a turbine 16 coupled to thecompressor 12, and a computer control system (controller) 18. An inletduct 20 to the compressor 12 may feed ambient air to compressor 12. Theinlet duct 20 may have ducts, filters, screens and noise abatementdevices that contribute to a pressure loss of ambient air flowingthrough the inlet 20 and into inlet guide vanes 21 of the compressor 12.An exhaust duct 22 for the turbine directs combustion gases from theoutlet of the turbine 10 through, for example, emission control andnoise abatement devices. The amount of inlet pressure loss and backpressure may vary over time due to the addition of components and due todust and dirt clogging the inlet 20 and exhaust ducts 22. The turbine 10may drive a generator 24 that produces electrical power.

As described in U.S. Patent Application Publication 2008/0243352, theoperation of the gas turbine 10 may be monitored by several sensors 26designed to measure different performance-related variables of theturbine 10, the generator and the ambient environment. For example,groups of redundant temperature sensors 26 may monitor ambienttemperature surrounding the gas turbine 10, compressor dischargetemperature, turbine exhaust gas temperature, and other temperaturemeasurements of the gas stream through the gas turbine 10. Similarly,groups of redundant pressure sensors 26 may monitor ambient pressure,and static and dynamic pressure levels at the compressor inlet andoutlet turbine exhaust, at other locations in the gas stream through thegas turbine 10. Groups of redundant humidity sensors 26, for example,wet and dry bulb thermometers, may measure ambient humidity in the inletduct of the compressor 12. Groups of redundant sensors 26 may alsoinclude flow sensors, speed sensors, flame detector sensors, valveposition sensors, guide vane angle sensors, or the like, that sensevarious parameters pertinent to the operation of gas turbine 10. As usedherein, “parameters” refer to items that can be used to define theoperating conditions of the turbine, such as but not limited totemperatures, pressures, and gas flows at defined locations in theturbine.

Also described in U.S. Patent Application Publication 2008/0243352A, thefuel control system 28 regulates the fuel flowing from a fuel supply tothe combustor 14, one or more splits between the fuel flowing intoprimary and secondary fuel nozzles, and the amount of fuel mixed withsecondary air flowing into a combustion chamber. The fuel control system28 may also select the type of fuel for the combustor. The fuel controlsystem 28 may be a separate unit or may be a component of the maincontroller 18. The controller 18 may be a computer system having atleast one processor that executes programs and operations to control theoperation of the gas turbine using sensor inputs and instructions fromhuman operators. The programs and operations executed by the controller18 may include, among others, sensing or modeling operating parameters,modeling operational boundaries, applying operational boundary models,applying scheduling algorithms, and applying boundary control logic toclose loop on boundaries. The commands generated by the controller 18may cause actuators on the gas turbine to, for example, adjust valves(actuator 27) between the fuel supply and combustors that regulate theflow, fuel splits and type of fuel flowing to the combustors; adjustinlet guide vanes 21 (actuator 29) on the compressor; adjust inlet bleedheat; as well as activate other control settings on the gas turbine.

U.S. Patent Application Nos. 2002/0106001 and 2004/0076218, the entirecontents of which are incorporated here by reference, describe a methodand system for adjusting turbine control algorithms to provide accuratecalculation of a firing temperature and combustion reference temperatureof a gas turbine as the water vapor content in a working fluid variessubstantially from a design value. These references disclose usingturbine temperature exhaust and turbine pressure ratio for controllingthe firing temperature.

However, the traditional methods and systems are limited in theircapability of controlling the gas turbine and accordingly, it would bedesirable to provide systems and methods that obtain a more accuratefiring temperature control, and/or a more accurate combustion parameterscontrol, and/or a more accurate exhaust emissions control.

SUMMARY

According to one exemplary embodiment, there is a method for controllingan operating point of a gas turbine that includes a compressor, acombustor and at least a turbine. The method includes a step ofdetermining a turbine exhaust pressure at an exhaust of the turbine; astep of measuring a compressor pressure discharge at the compressor; astep of determining a turbine pressure ratio based on the turbineexhaust pressure and the compressor pressure discharge; a step ofcalculating an exhaust temperature at the exhaust of the turbine as afunction of the turbine pressure ratio; a step of identifying areference exhaust temperature curve in a plane defined by the exhausttemperature and the turbine pressure ratio, wherein the referenceexhaust temperature curve includes those points that are optimal foroperating the gas turbine; and a step of controlling the gas turbine tomaintain the operating point on the reference exhaust temperature curve.

According to another exemplary embodiment, there is a gas turbine havinga control device for controlling an operating point of the gas turbine.The gas turbine includes a compressor configured to compress a fluid, acombustor connected to a discharge of the compressor and configured tomix the compressed fluid with fuel, at least a turbine connected to thecompressor and configured to expand burnt gas from the combustor togenerate power to an output of the gas turbine, a pressure sensorprovided at the discharge of the compressor to measure a compressorpressure discharge, and a processor that communicates with the pressuresensor. The processor is configured to determine a turbine exhaustpressure at an exhaust of the turbine, determine a turbine pressureratio based on the turbine exhaust pressure and the compressor pressuredischarge, calculate an exhaust temperature at the exhaust of theturbine as a function of the turbine pressure ratio, identify areference exhaust temperature curve in a plane defined by the exhausttemperature and the turbine pressure ratio, wherein the referenceexhaust temperature curve includes those points that are optimal foroperating the gas turbine, and control the gas turbine to maintain theoperating point on the reference exhaust temperature curve.

According to still another exemplary embodiment, there is a computerreadable medium including computer executable instructions, wherein theinstructions, when executed, implement a method for controlling anoperating point of a gas turbine that includes a compressor, a combustorand at least a turbine. The method includes a step of determining aturbine exhaust pressure at an exhaust of the turbine; a step ofmeasuring a compressor pressure discharge at the compressor; a step ofdetermining a turbine pressure ratio based on the turbine exhaustpressure and the compressor pressure discharge; a step of calculating anexhaust temperature at the exhaust of the turbine as a function of theturbine pressure ratio; a step of identifying a reference exhausttemperature curve in a plane defined by the exhaust temperature and theturbine pressure ratio, where the reference exhaust temperature curveincludes those points that are optimal for operating the gas turbine;and a step of controlling the gas turbine to maintain the operatingpoint on the reference exhaust temperature curve.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate one or more embodiments and,together with the description, explain these embodiments. In thedrawings:

FIG. 1 is a schematic diagram of a conventional gas turbine;

FIG. 2 is a schematic diagram of a gas turbine considered in anembodiment of the subject matter disclosed;

FIG. 3 is a graph illustrating the variation of an exhaust temperatureversus a pressure ratio of the turbine according to an exemplaryembodiment;

FIG. 4 is a schematic illustration of a relationship between operatingpoints and optimal operating points of the gas turbine according to anexemplary embodiment;

FIG. 5 is a schematic diagram of an exhaust temperature versus turbinepressure ratio plane according to an exemplary embodiment;

FIG. 6 is a schematic diagram of a reference exhaust temperature curvein the plane of FIG. 5 according to an exemplary embodiment;

FIG. 7 is a flow chart illustrating steps for calculating an exhausttemperature set point for the turbine according to an exemplaryembodiment;

FIG. 8 is a flow chart illustrating steps for controlling the gasturbine according to an exemplary embodiment; and

FIG. 9 is a schematic diagram of a controller used to control the gasturbine.

DETAILED DESCRIPTION

The following description of the exemplary embodiments refers to theaccompanying drawings. The same reference numbers in different drawingsidentify the same or similar elements. The following detaileddescription does not limit the invention. Instead, the scope of theinvention is defined by the appended claims. The following embodimentsare discussed, for simplicity, with regard to the terminology andstructure of a single shaft gas turbine system. However, the embodimentsto be discussed next are not limited to these systems, but may beapplied to other systems, for example multiple shaft gas turbines.

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with an embodiment is included inat least one embodiment of the subject matter disclosed. Thus, theappearance of the phrases “in one embodiment” or “in an embodiment” invarious places throughout the specification is not necessarily referringto the same embodiment. Further, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

As discussed above with regard to FIG. 1, various parameters of theturbine 10 may be measured and/or calculated for determining a desiredquantity to be monitored. Such a quantity is the firing temperature ofthe turbine. By maintaining the firing temperature of the turbine withinan optimum range, the operation of the turbine is considered to besmooth and under control. When the firing temperature of the turbineexits the optimum range, the controller 18 is configured to change, forexample, the compressor's air flow rate and thus, a compressor pressureratio, for adjusting the firing temperature. Events that may determinethe firing temperature to exit the optimum range is, for example, achange in a load of the gas turbine or a gas fuel composition change.

However, the novel embodiments to be discussed next do not rely on thetraditional paradigms for controlling the gas turbine but rather rely ona novel paradigm, e.g., controlling a turbine temperature exhaust basedon a turbine pressure ratio. This novel paradigm offers a more accurateestimation of the state of the gas turbine and is also more sensitive tochanges occurring in the functioning of the gas turbine, e.g., loadchange.

According to an exemplary embodiment, an exhaust temperature isdetermined as a function of the turbine pressure ratio and the exhausttemperature is monitored and maintained within certain boundaries forensuring an efficient operation of the gas turbine, e.g., accommodatingbase load, low load, high load, etc. More details about determining theexhaust temperature and the turbine pressure ratio are discussed nextwith regard to FIG. 2. FIG. 2 shows a gas turbine 30 having a compressor32 configured to receive a fluid (e.g., air) through an inlet duct 36.Sensors 34 may be disposed at inlet duct 36 for measuring at least oneof a pressure, temperature, humidity, etc.

The fluid is compressed by the compressor 32 and the compressed fluid issent to a combustor 40 via a path 42 to be mixed with fuel (e.g.,natural gas) supplied by a feeding duct 44. More sensors 34 may bedisposed in or around combustor 40 for measuring characteristics of thecompressed fluid and/or the fuel. A combustion in combustor 40 takesplace and this combustion raises the temperature of the mixture ofcompressed fluid and fuel to a firing temperature. Fuel is provided viafeeding duct 44 to primary and secondary burners, as disclosed later.Valves 45 a and 45 b are used to provide fuel to the primary andsecondary burners. A control unit 70 is also configured to adjust valves45 a and 45 b to provide a desired percentage of the fuel to the primaryand secondary valves. The flow of combusted gas, having a high energy,is supplied via ducts 52 to a turbine 50, which may be mechanicallyconnected with a shaft 56 to a generator 54. The generator 54 mayproduce electric power. Turbine 50 is also mechanically connected via ashaft 58 to the compressor 30, thus supplying the required driving powerto the compressor 30. Discharge gases are expelled from the turbine 50through an outlet duct 60. Both the inlet duct 52 and the outlet duct 60may be monitored by sensors 34.

Data from sensors 34 is provided to the control unit 70. The controlunit 70 may receive additional data via an input port 72. Based onprocesses computed by the control unit 70, various commands are providedvia an output port 74 to different parts of the gas turbine 30, e.g.,commands to rotate the vanes, to modify a rotational speed of the shaft,etc. A detailed structure of the control unit 70 is discussed later.

According to an exemplary embodiment, the proposed new control of thegas turbine is based on a turbine temperature exhaust (ttx), which ismeasured/determined at outlet 60, versus the turbine pressure ratio(tpr), which is measured/determined as a ratio between a dischargepressure of the compressor 32 and an exhaust pressure of the turbine 50.With reference to FIG. 2, the discharge pressure of the compressor 32 ismeasured at point 80 and the exhaust pressure of the turbine 50 ismeasured at point 60. However, according to an exemplary embodiment, theexhaust pressure may be measured/estimated inside combustor 40, at aninlet of turbine 50 or inside turbine 50. These pressures are discussedlater in more details. It is noted that the particulars discussed nextfor determining ttx are for illustrative purposes and not to limit thesubject matter disclosed.

FIG. 3 shows a (ttx, tpr) plane. Each point in this plane may beconsidered as belonging to a set A as shown in FIG. 4. Set A is definedas including operating points for the gas turbine 30 based on acombustion model. Set A includes a subset B of points. These points aredetermined as discussed next and are defined as optimum operating pointsfor the gas turbine 30.

Those points of the plane (ttx, tpr), i.e., points from set A, thatcorrespond to constant firing temperature, constant speed, constant IGVangle, constant air's specific humidity and constant bleed conditionsmay be represented by a curve 90, which may have an upwards concavity.The turbine pressure ratio tpr may vary with the compressor inlettemperature. An error introduced when approximating curve 90, which maybe a parabola with its osculatory straight line 92 at tpr=tpr₀ is smalland may be neglected for values of tpr near to tpr₀. One skilled in theart would recognize that other approximating functions may be used.

Varying gradually the compressor inlet temperature, the compressor speedand the IGV angle, the curve 90 changes gradually, for example, withoutany discontinuity in its prime derivative. Therefore, the constantfiring temperature locus, which may be calculated based on ttx, can beapproximated by the linear interpolation of the osculatory straight line92.

Based on the points in set B discussed above, a function f to bediscussed later, is applied to determine points belonging to a set C.The points of set C are set points for the gas turbine operation as percontrol logic. In other words, points belonging to set C are calculated,as discussed next, and the operator of the gas turbine 30 controls someparameters for maintaining the gas turbine within set C. FIG. 4illustrates this concept.

According to an exemplary embodiment, function f may be defined asf=g·h·l, where g, h, and l are mathematical functions or operators. Forexample, g may be a linear interpolation with an opportune fuelcharacteristic, h may be a bilinear interpolation of the angles of IGVand gas turbine speed, and l may be a polytropic correction given byp·T^(((1-γ)/γ))=constant. Setting the domain B, the codomain C isentirely defined through function f. Local perturbations in B producelocal perturbations in C. Depending on the application, more or lessfunctions or different functions may be used for defining function f. Inother words, instead of the g, h, and l functions discussed above, otherfunctions may be used or a different number of functions.

The determination of a set ttx temperatures, which is desired to bemaintained for an efficient operation of the gas turbine 30 is nowdiscussed. Assume that the gas turbine may operate in the followingranges: for an ambient temperature tamb, consider a rangetamb_(i-1)≦tamb≦tamb_(i), for an IGV angle igv, consider a rangeigv_(j-1)≦igv≦igv_(j), and for a gas turbine speed tnh, consider a rangetnh_(k-1)≦tnh≦tnh_(k). Also suppose that the gas turbine is controlledat optimum firing temperature. Based on the above ranges, operationalpoints of the gas turbine may be represented in the (ttx, tpr) spaceshown in FIG. 5 by curves defined by the following points. There arefour points A1 to A4 for a lean fuel and for the lowest ambienttemperature; there are four points B1 to B4 for the lean fuel and forthe highest ambient temperature; there are four points C1 to C4 for arich fuel and the lowest ambient temperature; and there are four pointsD1 to D4 for the rich fuel and the highest ambient temperature. Thenumber of points may vary according to the nature of the interpolatingfunction.

The lean fuel and the rich fuel are defined as follow. Gas turbines forindustrial applications are using natural gas that includes CH₄ morethan 90%. Natural gas is considered to be a rich gas fuel. Blending thenatural gas with inert gases, for example, nitrogen, carbon dioxide, andargon, produces leaner gas fuels, i.e., lower LHV value (LHV is thelower heating value of the gas and describes the amount of energy thatcan be obtained from a unit of mass of the gas by burning the gas). Therich fuel may be obtained by blending the natural gas with heavierhydrocarbons, like ethane, propane, and/or butane.

For each of the above discussed set of points, a central point (A5, B5,C5 and D5) is calculated using two bilinear interpolations (function gdiscussed above). A bilinear interpolation is an extension of linearinterpolation for interpolating functions of two variables on a regulargrid. The bilinear interpolation performs linear interpolation first inone direction, and then again in the other direction. Points A5 and B5define a temperature control curve 100 for the lean gas and points C5and D5 define a temperature control curve 102 for the rich gas. Asdiscussed above, another function than a bilinear interpolation may beused.

A ttx_(set point) is determined by using a linear interpolation(function h discussed above or, in another application, other functions)of the two ordinates corresponding to the actual pressure ratios on thetwo control curves 100 and 102, based on the LHV_(actual gas), theLHV_(rich gas) and the LHV_(lean gas).

If more points are calculated for other conditions and/or values of theconsidered parameters, more ttx_(set point) may be determined. Plottingthese points versus a corresponding tpr ratio results in a referenceexhaust temperature curve 104, which is shown in FIG. 6. It is notedthat the reference exhaust temperature curve 104 lies between the twocontrol curves 100 and 102. According to an exemplary embodiment (notillustrated), curve 104 is parallel to curves 100 and 102.

Steps for calculating the ttx_(set point) may be represented in theblock diagram shown in FIG. 7. According to this figure, data selectorunit 110 receives as input the ambient temperature tamb, the rotationangle of the vanes IGV, the rotational speed tnh of the shaft and therich gas matrix data. An example of the rich gas matrix data is:

ttxr

tamb_(i) ttxr_(i, j, k) igv₁ igv₂ . . . igv₅ igv₆ tnh₁ ttxr_(i, 1, 1)ttxr_(i, 2, 1) . . . ttxr_(i, 5, 1) ttxr_(i, 6, 1) tnh₂ ttxr_(i, 1, 2) .. . . . . . . . . . . tnh₃ ttxr_(i, 1, 3) . . . . . . . . . . . . tnh₄ttxr_(i, 1, 4) . . . . . . . . . ttxr_(i, 6, 4)and the turbine pressure ratio matrix for rich gas is given by:tprr

tamb_(i) tprr_(i, j, k) igv₁ igv₂ igv₃ igv₅ igv₆ tnh₁ tprr_(i, 1, 1)tprr_(i, 2, 1) tprr_(i, 3, 1) tprr_(i, 5, 1) tprr_(i, 6, 1) tnh₂tprr_(i, 1, 2) . . . . . . . . . . . . tnh₃ tprr_(i, 1, 3) . . . . . . .. . . . . tnh₄ tprr_(i, 1, 4) . . . . . . . . . tprr_(i, 6, 4)

Eight points C1 to C4 and D1 to D4 (shown in FIG. 5) are output by thedata selector unit 110. This output is provided as input to interpolatorunit 112. The same process is repeated by data selector unit 114 for thesame parameters except that a lean gas matrix data is used instead ofthe rich gas matrix data. Output from interpolators 112 and 116, i.e.,rich gas ttx versus tpr actual control curve and lean gas ttx versus tpractual control gas, are provided as input to calculation unit 118 forcalculating two ttx set points. Linear interpolator 120 receives the twottx set points and interpolates them to produce a final point, thettx_(set point). Based on the output of the linear interpolator 120, afiring unit 122 may calculate variations of the ttx_(set point) of thegas turbine. It is noted that the linear interpolator 120 and the firingunit 122 may receive directly information about the fuel gas LHV.

Having the ttx_(set point), controller 70 may be programmed to monitorthis value and to adjust various parameters of the gas turbine 30 (e.g.,angle of IGV, fuel amount, etc.) to maintain the ttx_(set point) in apredetermined range for an efficient operation of the gas turbine. Inone exemplary embodiment in which a single shaft gas turbine is used,the ttx_(set point) may be adjusted by controlling the IGV angle. Thereference exhaust temperature curve ttxh 104, which the gas turbine isdesired to follow, is now calculated.

Consider three vectors that identify the gas turbine operatingparameters. These vectors are tamb, igv, and tnh, and they correspond tothe ambient temperature, angle of IGV vanes, and shaft's rotationalspeed. The mathematical expressions for these three vectors are:

tamb=[tamb_(i)]=[tamb₁, tamb₂, . . . , tamb₇]

with index i being:2 if tamb<tamb₂3 if tamb₂≦tamb<tamb₃4 if tamb₃≦tamb<tamb₄5 if tamb₄≦tamb<tamb₅6 if tamb₅≦tamb<tamb₆7 if tamb₆≦tamb,where tamb is the actual ambient temperature.

The igv angle vector is defined as:

igv=[igv_(j)]=[igv₁, igv₂, . . . , igv₆] with index j being:

2 if igv<igv₂3 if igv₂≦igv<igv₃4 if igv₃≦igv<igv₄5 if igv₄≦igv<igv₅6 if igv₅≦igv,where igv is the actual igv angle.

The tnh shaft speed vector is defined as:

tnh=[tnh_(k)]=[tnh₁, tnh₂, tnh₃, tnh₄] with index k being:

2 if tnh<tnh₂3 if tnh₂≦tnh<tnh₃4 if tnh₃≦tnh,where tnh is the actual shaft speed percentage. The values for i, j, andk differ from application to application and may include a large numberof possibilities.

Four 3D matrices are introduced for calculating the reference exhausttemperature curve ttxh, i.e., a reference curve used by the operator forcontrolling the gas turbine. According to an exemplary embodiment, ttxhcan be seen as a locus of points where the gas turbine operates atoptimal ttx and tpr values. The four matrices are the exhausttemperature lean fuel matrix ttxl, the pressure ratio lean fuel matrixtprl, the exhaust temperature rich fuel matrix ttxr, and the pressureratio rich fuel tprr. Elements of these matrices are listed below:

ttxl=[ttxl_(i,j,k)] for lean fuel,tprl=[tprl_(i,j,k)] for lean fuel,ttxr=[ttxr_(i,j,k)] for rich fuel, andtprr=[tprr_(i,j,k)] for rich fuel.

Assuming that the actual operating conditions tamb, igv and tnh arewithin ranges tamb_(i-1)≦tamb<tamb_(i); igv_(j-1)≦igv<igv_(j); andtnh_(k-1)≦tnh<tnh_(k), the actual reference curve ttxh is given by

ttxh=ttxha+Δttxh,

where ttxha defines a reference curve for the operation of the gasturbine at optimal ttx and tpr points, but also taking into accountcompressor inlet pressure and gas turbine exhaust pressure drop, andΔttxh is a correction of ttxha that is used to maintain the turbinefiring temperature at optimum values while the inlet and exhaustpressure drops of the turbine vary.

Reference curve ttxha is defined as

ttxha=ttxhr·(LHV−LHVl)/(LHVr−LHVl)+ttxhl·(LHVr−LHV)/(LHVr−LHVl),

where the parameters defining ttxha are defined as follows:

ttxhr=ttxr _(i-1)+(ttxr _(i) −ttxr _(i-1))/(tprr _(i) −tprr_(i-1))·(tpr−tprr _(i-1)),

ttxhl=ttxl _(i-1)+(ttxl _(i) −ttxl _(i-1))/(tprl _(i) −tprl_(i-1))·(tpr−tprl _(i-1)),

LHV is the lower heating value of the actual fuel,LHVl is the lower heating value of the lean fuel,LHVr is the lower heating value of the rich fuel.

The following bilinear interpolations are applied:

ttxl _(i-1)=BilinearInterpolation(ttxl _(i-1,j-1,k-1) , ttxl_(i-1,j,k-1) , ttxl _(i-1,j,k) , ttxl _(i-1,j-1,k) , igv, tnh)=ttxl_(i-1,j-1,k-1)(igv _(j) −igv)/(igv _(j) −igv _(j-1))·(tnh _(k)−tnh)/(tnh _(k) −tnh _(k-1))+ttxl _(i-1,j,k-1)·(igv−igv _(j-1))/(igv_(j) −igv _(j-1))·(tnh _(k) −tnh)/(tnh _(k) −tnh _(k-1))+ttxl_(i-1,j,k)·(igv−igv _(j-1))/(igv _(j) −igv _(j-1))·(tnh−tnh _(k-1))/(tnh_(k) −tnh _(k-1))+ttxl _(i-1,j-1,k)·(igv _(j) −igv)/(igv _(j) −igv_(j-1))·(tnh−tnh _(k-1))/(tnh _(k) −tnh _(k-1)),

ttxl _(i)=BilinearInterpolation(ttxl _(i-1,j-1,k-1) , ttxl _(i-1,j,k-1), ttxl _(i-1,j,k) , ttxl _(i-1,j-1,k) , igv, tnh)=ttxl_(i-1,j-1,k-1)·(igv _(j) −igv)/(igv _(j) −igv _(j-1))·(tnh _(k)−tnh)/(tnh _(k) −tnh _(k-1))+ttxl _(i-1,j,k-1)·(igv−igv _(j-1))/(igv_(j) −igv _(j-1))·(tnh _(k) −tnh)/(tnh _(k) −tnh _(k-1))+ttxl_(i-1,j,k)·(igv−igv _(j-1))/(igv _(j) −igv _(j-1))·(tnh−tnh _(k-1))/(tnh_(k) −tnh _(k-1))+ttxl _(i-1,j-1,k)·(igv _(j) −igv)/(igv _(j) −igv_(j-1))·(tnh−tnh _(k-1))/(tnh _(k) −tnh _(k-1)),

tprl _(i-1)=BilinearInterpolation(tprl _(i-1,j-1,k-1) , tprl_(i-1,j,k-1) , tprl _(i-1,j,k) , tprl _(i-1,j-1,k) , igv, tnh)=tprl_(i-1,j-1,k-1)·(igv _(j) −igv)/(igv _(j) −igv _(j-1))·(tnh _(k)−tnh)/(tnh _(k) −tnh _(k-1))+tprl _(i-1,j,k-1)·(igv−igv _(j-1))/(igv_(j) −igv _(j-1))·(tnh _(k) −tnh)/(tnh _(k) −tnh _(k-1))+tprl_(i-1,j,k)·(igv−igv _(j-1))/(igv _(j) −igv _(j-1))·(tnh−tnh _(k-1))/(tnh_(k) −tnh _(k-1))+tprl _(i-1,j-1,k)·(igv _(j) −igv)/(igv _(j) −igv_(j-1))·(tnh−tnh _(k-1))/(tnh _(k) −tnh _(k-1)),

tprl _(i)=BilinearInterpolation(tprl _(i-1,j-1,k-1) , tprl _(i-1,j,k-1), tprl _(i-1,j,k) , tprl _(i-1,j-1,k) , igv, tnh)=tprl_(i-1,j-1,k-1)·(igv _(j) −igv)/(igv _(j) −igv _(j-1))·(tnh _(k)−tnh)/(tnh _(k) −tnh _(k-1))+tprl _(i-1,j,k-1)·(igv−igv _(j-1))/(igv_(j) −igv _(j-1))·(tnh _(k) −tnh)/(tnh _(k) −tnh _(k-1))+tprl_(i-1,j,k)·(igv−igv _(j-1))/(igv _(j) −igv _(j-1))·(tnh−tnh _(k-1))/(tnh_(k) −tnh _(k-1))+tprl _(i-1,j-1,k)·(igv _(j) −igv)/(igv _(j) −igv_(j-1))·(tnh−tnh _(k-1))/(tnh _(k) −tnh _(k-1)),

ttxr _(i-1)=BilinearInterpolation(ttxr _(i-1,j-1,k-1) , ttxr_(i-1,j,k-1) , ttxr _(i-1,j,k) , ttxr _(i-1,j-1,k) , igv, tnh)=ttxr_(i-1,j-1,k-1)·(igv _(j) −igv)/(igv _(j) −igv _(j-1))·(tnh _(k)−tnh)/(tnh _(k) −tnh _(k-1))+ttxr _(i-1,j,k-1)·(igv−igv _(j-1))/(igv_(j) −igv _(j-1))·(tnh _(k) −tnh)/(tnh _(k) −tnh _(k-1))+ttxr_(i-1,j,k)·(igv−igv _(j-1))/(igv _(j) −igv _(j-1))·(tnh−tnh _(k-1))/(tnh_(k) −tnh _(k-1))+ttxr _(i-1,j-1,k)·(igv _(j) −igv)/(igv _(j) −igv_(j-1))·(tnh−tnh _(k-1))/(tnh _(k) −tnh _(k-1)),

ttxr _(i)=BilinearInterpolation(ttxr _(i-1,j-1,k-1) , ttxr _(i-1,j,k-1), ttxr _(i-1,j,k) , ttxr _(i-1,j-1,k) , igv, tnh)=ttxr_(i-1,j-1,k-1)·(igv _(j) −igv)/(igv _(j) −igv _(j-1))·(tnh _(k)−tnh)/(tnh _(k) −tnh _(k-1))+ttxr _(i-1,j,k-1)·(igv−igv _(j-1))/(igv_(j) −igv _(j-1))·(tnh _(k) −tnh)/(tnh _(k) −tnh _(k-1))+ttxr_(i-1,j,k)·(igv−igv _(j-1))/(igv _(j) −igv _(j-1))·(tnh−tnh _(k-1))/(tnh_(k) −tnh _(k-1))+ttxr _(i-1,j-1,k)·(igv _(j) −igv)/(igv _(j) −igv_(j-1))·(tnh−tnh _(k-1))/(tnh _(k) −tnh _(k-1)),

tprr _(i-1)=BilinearInterpolation(tprr _(i-1,j-1,k-1) , tprr_(i-1,j,k-1) , tprr _(i-1,j,k) , tprr _(i-1,j-1,k) , igv, tnh)=tprr_(i-1,j-1,k-1)·(igv _(j) −igv)/(igv _(j) −igv _(j-1))·(tnh _(k)−tnh)/(tnh _(k) −tnh _(k-1))+tprr _(i-1,j,k-1)·(igv−igv _(j-1))/(igv_(j) −igv _(j-1))·(tnh _(k) −tnh)/(tnh _(k) −tnh _(k-1))+tprr_(i-1,j,k)·(igv−igv _(j-1))/(igv _(j) −igv _(j-1))·(tnh−tnh _(k-1))/(tnh_(k) −tnh _(k-1))+tprr _(i-1,j-1,k)·(igv _(j) −igv)/(igv _(j) −igv_(j-1))·(tnh−tnh _(k-1))/(tnh _(k) −tnh _(k-1)),

tprr _(i)=BilinearInterpolation(tprr _(i-1,j-1,k-1) , tprr _(i-1,j,k-1), tprr _(i-1,j,k) , tprr _(i-1,j-1,k) , igv, tnh)=tprr_(i-1,j-1,k-1)·(igv _(j) −igv)/(igv _(j) −igv _(j-1))·(tnh _(k)−tnh)/(tnh _(k) −tnh _(k-1))+tprr _(i-1,j,k-1)·(igv−igv _(j-1))/(igv_(j) −igv _(j-1))·(tnh _(k) −tnh)/(tnh _(k) −tnh _(k-1))+tprr_(i-1,j,k)·(igv−igv _(j-1))/(igv _(j) −igv _(j-1))·(tnh−tnh _(k-1))/(tnh_(k) −tnh _(k-1))+tprr _(i-1,j-1,k)·(igv _(j) −igv)/(igv _(j) −igv_(j-1))·(tnh−tnh _(k-1))/(tnh _(k) −tnh _(k-1)),

The correction Δttxh is given by:

Δttxh=ttxh·((pamb _(actual) +Δp _(exhaust ref))/(pamb _(actual) +Δp_(exhaust)))^((γ/(1-γ))-1))+((pamb _(actual) −Δp _(inlet ref))/(pamb_(actual) −Δp _(inlet)))^((γ/(1-γ))-1))),

where γ=a·tpr+b with a and b being constants and γ is made to fit thegas turbine polytropic expansion (p·t^(((1-γ)/γ))=constant).

The correction Δttxh takes into account, among other things, the actualgas turbine exhaust and inlet pressure drops. As the gas turbinetemperature control curves (ttxh for example) depend on a referenceexhaust pressure drop Δp_(exhaust ref) and reference inlet pressure dropΔp_(inlet ref), it is possible to correct these curves for differentexhaust and inlet pressure drops by using, for example, function Δttxh.

The actual inlet pressure drop value Δp_(inlet act) may be measuredinstead of being estimated due to the amount of dirt at the input of thecompressor. In other words, the compressor inlet system pressure dropdepends on the flow conditions and on the dirt in the inlet filter, theperiodic dirt deposition and removal may cause an unpredictablevariability of the inlet pressure drop over the time. In an application,if the LHV signal is not available, for example due to calorimeter faultor calibration problems, the controller 70 may be configured to use aLHV_(default) to override the actual LHV.

The above bilinear interpolations, linear interpolation and polytropicexpansion, when applied as indicated above to the parameters of the gasturbine, for example, IGV angles and shaft rotational speed at variouspoints i, j, and k of the allowed ranges, generate the ttx_(set point)on the reference curve ttxh. In one exemplary embodiment, multiplettx_(set point) are calculated for the gas turbine for variousconditions and all these points ttx_(set point) are part of the ttxhcurve. Other reference curves may be determined from ttxh, as discussednext. These additional reference curves may also be used to control theoperation of the gas turbine.

According to an exemplary embodiment, a reference exhaust temperatureversus compressor pressure ratio curve TTRX may be used to control thegas turbine. The TTRX curve may be defined as TTRX=Min(Isotherm_(NO),ttxh), where Isotherm_(NO) is defined as an isotherm of the gas turbineat normal operating conditions. In one application, the Isotherm_(NO)represents the maximum temperature to which the rotor of the turbine maybe exposed. A control curve for the exhaust temperature versus IGV maybe defined as TTRXGV=TTRX. A control curve for the exhaust temperatureversus fuel may be defined as TTRXB=TTRXB_(NO) if a peak load mode isoff and TTRXB=TTRXB_(PK) if the peak load mode is on. The peak load modeis defined as a gas turbine that runs at constant operating conditions(ambient temperature, pressure, shaft speed, IGV position, and fuel gascomposition) and delivers a power higher than the nominal one. Thiscondition occurs when the gas turbine's operating firing temperature ishigher than the nominal temperature. TTRXB_(NO) is given byTTRX+Min((IGV_(max)−IGV_(set point))·Δ1, Δ2), where Δ2 is a value thatlimits the value of the Min function, and TTRXB_(PK) is given byMin(Isotherm_(PK), ttxh+ΔPK).

ΔPK is given by

ΔPK=Δttxr·(LHV−LHVl)/(LHVr−LHVl)+Δttxl·(LHVr−LHV)/(LHVr−LHVl), with LHVbeing the lowest heating value of the actual fuel,

LHVI being the lowest heating value of the lean fuel,LHVr being the lowers heating value of the rich fuel,

Δttxl=Δttxl ^(i-1)+(Δttxl _(i) −Δttxl _(i-1))·(tamb−tamb _(i-1))/(tamb_(i) −tamb _(i-1)), and

Δttxr=Δttxr _(i-1)+(Δttxr _(i-1) −Δttxr _(i-1))·(tamb−tamb _(i-1))/(tamb_(i) −tamb _(i-1)).

The above exhaust temperature control via IGV and exhaust temperaturecontrol via fuel curves may be used in the control of the gas turbine asfollows. A gas turbine may be controlled by varying, for example, thespeed of the shaft of the turbine, the angle of IGV (that directlycontrols the amount of air provided to the compressor), the amount offuel provided to the combustor, the ratio of fuel/air provided to thecombustor, etc. According to an exemplary embodiment, for a single shaftgas turbine, the angle of the IGV is used first to control the operationof the gas turbine, i.e., to maintain the ttx_(act point) on the ttxhcurve calculated above (in the ttx versus tpr plane). In other words,when the actual ttx_(act point) deviates from the ttxh curve due tovarious conditions of the gas turbine (e.g., change in load), a firstcontrol adjusts the angle of IGV for bringing the ttx_(act point) of thegas turbine to the ttx_(set point). However, this control may reach asaturation point, i.e., a point at which the angle of IGV may not bemodified further or it is not desired to be modified further. At thispoint, an amount of fuel to be provided to the gas turbine may be varieduntil the ttx_(act point) is made to coincide with the ttx_(set point).If this control becomes saturated, it is possible to change a ratiobetween the fluid provided by the compressor and the fuel injected intothe combustor, thus limiting the fuel flow rate and further regulatingttx_(act point).

To fully determine the ttxh curve in the ttx versus tpr plane, it isnext discussed the determination of the turbine pressure ratio tpr. Thegas turbine exhaust pressure is less difficult to be estimated than tobe measured. Although the pressures involved in the turbine pressureratio tpr may be measured, it is preferred to calculate tpr as discussednext because it is more accurate than the measured tpr. In this regard,it is noted that vortices may appear at locations 80 and 60 in the gasturbine, which make the measured pressures less accurate as they mayvary across a small distance. The estimation may be performed based oncharacteristics of a flue pressure drop, exhaust gas data and ambientpressure. According to an exemplary embodiment, the turbine pressureratio tpr is determined based on the estimated exhaust pressure drop andthe absolute compressor discharge pressure. In one embodiment, theexhaust pressure drop is determined at point 60 (see FIG. 2) while theabsolute compressor discharge pressure is determined at point 80 (seeFIG. 2). In another embodiment, for a compressor having multiple stages,the absolute compressor discharge pressure is determined after thedischarge diffuser, which is downstream of the last stage. According tothis exemplary embodiment, the absolute compressor discharge pressure ismeasured.

According to an exemplary embodiment, the exhaust pressure drop is madeup by two terms, the pressure drop due to a mass flowing in the flue ofthe turbine 50 and a pressure recovery due to a chimney effect. Thechimney effect may appear if there is a height difference between thegas turbine exhaust and the flue discharge to the atmosphere. The firstterm is given by a_(a)·ρ_(exhaust)·v² and the second term is given by(ρ_(air)−ρ_(exhaust))·Δh. The meaning of each constant, parameter andvariable used in the present calculations is provided later. Thus, thetotal exhaust pressure drop due to the flowing mass in the flue can beexpressed as:

Δp _(exhaust) =a _(a) ·p _(exhaust) ·v ²−(ρ_(air)−ρ_(exhaust))·Δh, whichmay be rewritten as

a _(a)·ρ_(exhaust) ·v ² =a _(a)·ρ_(exhaust)·(W _(exhaust)/(ρ_(exhaust)·a _(b)))² =a _(a)·(W _(exhaust) /a _(b))²/ρ_(exhaust) =a/ρ _(exhaust)·W _(exhaust) ².

To simplify this expression, assume that the density ρ of the gas in theflue is independent of the actual exhaust pressure drop and depends onlyon the discharge pressure, which here is the ambient pressure, as theexhaust pressure drop is assumed to be only a small fraction of theambient pressure. Thus, the error introduced by this simplification canbe neglected. The exhaust gas density ρ_(exhaust) can be expressed as:

ρ_(exhaust)=ρ_(exhaust ref) ·ttx _(ref) /ttx _(act) ·pamb _(act) /pamb_(ref).

The ambient air density can be expressed as:

ρ_(air)=ρ_(air ref) ·tamb _(ref) /tamb _(act) ·pamb _(act) /pamb _(ref),

where:ρ_(exhaust) is the density of the exhaust gas at the ttx_(act)temperature and pamb_(act) ambient pressure,ρ_(exhaust ref) is the density of the exhaust gas at the ttx_(ref)temperature and pamb_(ref) ambient pressure,ρ_(air) is the density of the ambient air at the actual pressure andtemperature,ρ_(air ref) is the density of the ambient air at the reference pressureand temperature,Δh is the elevation difference between the gas turbine exhaust and theflue discharge to the atmosphere,v is the exhaust speed inside the flue,ttx_(ref) is the reference exhaust temperature,ttx_(act) is the actual exhaust temperature,pamb_(ref) is the reference ambient pressure,pamb_(act) is the actual ambient pressure,Wexhaust_(act) is the actual exhaust gas mass flow rate, anda is a constant typical for the specific exhaust duct.

It is assumed in this exemplary embodiment that the exhaust gascomposition is substantially constant over a premixed mode operation,and thus, its density is substantially constant at a given temperature.

The exhaust gas mass flow rate may be estimated as follows. Assume thatthe compressor air mass flow rate is independent of the compressorpressure ratio as the error introduced by this assumption is negligiblefor the purpose of the exhaust pressure drop estimation. The gasturbine's axial compressor air mass flow rate can be estimated by thefollowing transfer function:

Wair_(act) =SG _(ha) ·pinlet_(act) /pinlet_(ref)·(f ₃ ·x ³ +f ₂ ·x ² +f₁ ·x+f ₀)·f ₄ ·Wair_(ref) ·k, where

f ₀ =a ₀ ·y ³ +b ₀ ·y ² +c ₀ ·y,

f ₁ =a ₁ ·y ³ +b ₁ ·y ² +c ₁ ·y,

f ₂ =a ₂ ·y ³ +b ₁ ·y ² +c ₂ ·y,

f ₃ =a ₃ ·y ³ +b ₁ ·y ² +c ₃ ·y,

f ₄ =a ₄₁ ·z ³ +b ₄₁ ·z ² +c ₄₁ ·z+d ₄₁ if tnh _(act) /tnh _(ref) <tnh_(threshold),

a ₄₂ ·z ³ +b ₄₂ ·z ² +c ₄₂ ·z+d ₄₂ if tnh _(act) /tnh _(ref) ≧tnh_(threshold),

x=igv _(act) /igv _(ref),

y=tnh _(act) /tnh _(ref)·(tinlet_(ref) /tinlet_(act))^(0.5),

z=tnh _(act) /tnh _(ref)·(tinlet_(ref) /tinlet_(act)), and

a_(i) and a_(ij) are application-specific constants.

Because the gas turbine is equipped with an IBH system, at some partialload operating conditions, a fraction of the compressor's air mass flowrate is recirculated and does not enter the exhaust duct. Moreover, thefuel gas mass flow rate enters entirely through the exhaust duct.Therefore, Wexhaust_(act)=Wair_(act)·(1−IBH_(fraction))+Wfuel_(act). Inthis exemplary embodiment, it has been assumed that the air to thebearings compensates the air from the cooling blowers.

Considering that while the gas turbine is on exhaust temperature controland the fuel/air mass ratio is substantially constant for a specificfuel gas composition, the fuel/air mass flow ratio may be evaluated asfollows:

fa _(ratio) =Wfuel_(act) /Wair_(act) =Wfuel_(ref)/Wair_(ref)·LHV_(ref)/LHV_(act) =fa _(ratio ref)·LHV_(ref)/LHV_(act).

The IBH_(fraction) is a set point generated by the control panel andcontrolled while the system is not at fault. Then, the exhaust mass flowrate may be evaluated as:

Wexhaust_(act) =Wair_(act)·(1−IBH_(fraction))·(1+fa_(ratio ref)·LHV_(ref)/LHV_(act)).

The specific gravity of humid air SG_(ha) can be evaluated based on thespecific humidity as follows:

SG _(ha)=ρ_(ha)/ρ_(da),

m _(ha) =m _(da) +m _(wv),

m _(da) =m _(ha)·(1−sh),

m _(wv) =m _(ha) ·sh, and

V _(ha) =m _(ha)/ρ_(ha) =m _(da)/ρ_(da) +m _(wv)/ρ_(wv),

Multiplying this last expression by ρ_(ha), the following equation isobtained:

m _(ha) =m _(da)·ρ_(ha)/ρ_(da) +m _(wv)·ρ_(ha)/ρ_(wv), where

ρ_(ha)/ρ_(da) =SG _(ha) and ρ_(ha)/ρ_(wv)=ρ_(ha)·ρ_(da)/ρ_(da)·ρ_(wv)=SG _(ha) /SG _(wv).

Thus,

m _(ha) =m _(da)·ρ_(ha)/ρ_(da) +m _(wv)·ρ_(ha) /p _(wv) =m _(da) ·SG_(ha) +m _(wv) ·SG _(ha) /SG _(wv), or

m _(ha)=(1−sh)·m _(ha) ·SG _(ha) +sh·m _(ha) ·SG _(ha) /SG _(wv).

Dividing this last expression by m_(ha)

1=(1−sh)·SG _(ha) +sh·SG _(ha) /SG _(wv), or

SG _(wv) =SG _(ha)·((1−sh)·SG _(wv) +sh).

Finally,

SG _(ha) =SG _(wv)/((1−sh)·SG _(wv) +sh).

If the specific humidity signal is not available or the transmitter isin a fault mode, the specific humidity signal can be substituted by acurve of specific humidity versus ambient temperature generated by theinterpolation of the data shown in Table 1:

TABLE 1 shdefault Average air specific humidity vs. ambient temperaturetamb tamb₁ tamb₂ . . . . . . . . . tamb₆ tamb₇ sh_(i) sh₁ sh₂ . . . . .. . . . sh₆ sh₇

The following notations have been used in the above calculations:

pinlet_(act) is the actual air pressure at the compressor inlet,pinlet_(ref) is the reference air pressure at the compressor inlet,tamb is the ambient temperature,tinlet_(act) is the actual air temperature at the compressor inlet, maybe measured with at least two thermocouples such that the maximumreading of the thermocouples is considered to be tinlet_(act) or in casethat one thermocouple is faulty and/or the difference in readings is toolarge (for example 10 F), tamb is considered to be tinlet_(act),tinlet_(ref) is the reference air temperature at the compressor inlet,tnh_(act) is the compressor actual speed,tnh_(ref) is the compressor reference speed,igv_(act) is the actual igv angle,igv_(ref) is the reference igv angle,Wair_(act) is the actual air mass flow rate at the compressor inlet,Wair_(ref) is the reference air mass flow rate at the compressor inlet,Wexhaust_(act) is the actual exhaust gas mass flow rate,Wfuel_(act) is the fuel mass flow rate,IBH_(fraction) is the fraction of air bled from the compressordischarge,fa_(ratio ref) is the reference fuel air mass ratio,LHV_(ref) is the reference gas fuel's LHV,LHV_(act) is the actual gas fuel's LHV,sh is the air specific humidity,SG_(xx) is the specific gravity of xx (see subscript list below),ρ_(xx) is the density of xx (see subscript list below),m_(xx) is the mass of xx (see subscript list below),Vxx volume of xx (see subscript list below),ha is humid air,wv is water vapor, andda is dry air.

Having calculated the specific gravity, the mass flow rate through thecompressor and other parameters as discussed above, it is now possibleto calculate the turbine pressure ratio tpr. The algorithm forcalculating tpr may be summarized as follows:

-   -   calculate SG_(ha) to be SG_(wv)/((1−sh)·SG_(wv)+sh) if sh signal        is valid and available and sh_(default) if sh transmitter signal        fault;    -   assume x=igv_(act)/igv_(ref),        y=tnh_(act)/tnh_(ref)·(tinlet_(ref)/tinlet_(act))^(0.5), and        z=tnh_(act)/tnh_(ref)·(tinlet_(ref)/tinlet_(act));    -   f₀=a₀·y³+b₀·y²+c₀·y,    -   f₁=a₁·y³+b₁·y²+c₁·y,    -   f₂=a₂·y³+b₁·y²+c₂·y,    -   f₃=a₃·y³+b₁·y²+c₃·y,    -   f₄=a₄₁·z³+b₄₁·z²+c₄₁·z+d₄₁ if        tnh_(act)/tnh_(ref)<tnh_(threshold), and    -   a₄₂·z³+b₄₂·z²+c₄₂·z+d₄₂ if tnh_(act)/tnh_(ref)≧tnh_(threshold);    -   define        Wair_(act)=SG_(ha)·pinlet_(act)/pinlet_(ref)·(f₃·x³+f₂·x²+f₁·x+f₀)·f₄·Wair_(ref)·k,    -   evaluate        Wexhaust_(act)=Wair_(act)·(1−IBH_(fraction))·(1+fa_(ratio ref)·LHV_(ref)/LHV_(act)),    -   calculate        ρ_(air)=ρ_(air ref)·tamb_(ref)/tamb_(act)·pamb_(act)/pamb_(ref),    -   calculate        ρ_(exhaust)=ρ_(exhaust ref)·ttx_(ref)/ttx_(act)·pamb_(act)/pamb_(ref).    -   calculate        Δp_(exhaust)=a_(a)·ρ_(exhaust)·v²−(ρ_(air)−ρ_(exhaust))·Δh, and    -   evaluate tpr=cpd/(pamb_(act)+Δp_(exhaust)), where cpd is the        absolute compressor discharge pressure, which is measured in        this application.

Thus, the ttxh curve 104 (see FIG. 6) is fully determined at this stage.If the temperature control curves for the gas turbine have been set upfor a reference exhaust pressure drop Δp_(exhaust ref) and referenceinlet pressure drop Δp_(inlet ref), it is possible to correct thetemperature control curves for a different exhaust and inlet pressuredrops, for example, the actual one, by using the correction Δttxh, asalready discussed above.

One or more advantages of the temperature control logic described aboveare discussed now. Because the entire procedure developed above forcontrolling the gas turbine is matrix based, the procedure is flexibleand allows for easy site tuning. The procedure may bias the controlledexhaust temperature, during normal and peak load operation, based on theactual fuel's LHV (or other fuel characteristics if differentlyspecified). Based on this bias, it is possible to better controlpollutant emission, combustion dynamics and combustor's turn downmargins.

When the peak mode is enabled, the gas turbine may stay at normal firingtemperature if the base load power is enough to cover the drivenmachine's power demand and the gas turbine may stay in over-firing ifthe base load power does not cover the driven machine's power demand.The peak firing value may be biased by the fuel characteristics. Basedon this “smart” behavior, maintaining the peak mode always enabled, itis possible to configure the gas turbine to be more reactive in case ofa variation of a modified Wobbe index (MWI) base load, and/or toundertake a larger load step up starting from any operating point(largest spinning reserve).

The MWI is given by LHV_(gas)/(SG_(gas)·T_(gas))^(0.5), with LHV_(gas)being the lower heating value of the gas, SG_(gas) the specific gravityof gas, and T_(gas) the temperature of the fuel gas.

According to an exemplary embodiment, illustrated in FIG. 8, there is amethod for controlling an operating point of a gas turbine that includesa compressor, a combustor and at least a turbine. The method includes astep 800 of determining a turbine exhaust pressure at an exhaust of theturbine, a step 802 of measuring a compressor pressure discharge at thecompressor, a step 804 of determining a turbine pressure ratio based onthe turbine exhaust pressure and the compressor pressure discharge, astep 806 of calculating an exhaust temperature at the exhaust of theturbine as a function of the turbine pressure ratio, a step 808 ofidentifying a reference exhaust temperature curve in a plane defined bythe exhaust temperature and the turbine pressure ratio, where thereference exhaust temperature curve includes those points that areoptimal for operating the gas turbine, and a step 810 of controlling thegas turbine to maintain the operating point on the reference exhausttemperature curve.

Alternatively, the following steps may also be performed for controllingthe turbine. The gas turbine may have inlet guide vanes disposed at aninlet of the compressor and configured to adjust an amount of the fluidentering the compressor to maintain the operating point on the referenceexhaust temperature curve. The processor of the gas turbine may beconfigured to use the reference exhaust temperature curve to control afiring temperature of the combustor, where the firing temperature istemperature of the combustion products downstream a first stage nozzleof the turbine, or calculate a turbine exhaust pressure due to a massflowing in the flue of the turbine, and calculate a pressure recoverydue to a chimney effect, wherein the chimney effect is due to anelevation difference between the exhaust of the turbine and a fluedischarge to atmosphere, and add together the turbine exhaust pressuredue to the mass flowing and the pressure recovery to obtain the turbineexhaust pressure.

The turbine exhaust pressure may depends on ρ_(exhaust), which is adensity of the exhaust gas at an actual exhaust temperature and actualambient pressure, ρ_(exhaust ref), which is a density of the exhaust gasat the reference exhaust temperature and a reference ambient pressure,ρ_(air), which is a density of the ambient air at the actual pressureand temperature, ρ_(air ref), which is a density of the ambient air atthe reference pressure and temperature, Δh, which is the elevationdifference between the gas turbine exhaust and the flue discharge to theatmosphere, v, which is the exhaust speed inside the flue, ttx_(ref),which is the reference exhaust temperature, ttx_(act), which is theactual exhaust temperature, pamb_(ref), which is the reference ambientpressure, pamb_(act), which is the actual ambient pressure,pinlet_(act), which is the actual air pressure at the compressor inlet,pinlet_(ref), which is the reference air pressure at the compressorinlet, tamb, which is the ambient temperature, tinlet_(act), which isthe actual air temperature at the compressor inlet, tinlet_(ref), whichis the reference air temperature at the compressor inlet, tnh_(act),which is the compressor actual speed, tnh_(ref), which is the compressorreference speed, igv_(act), which is the actual igv angle, igv_(ref),which is the reference igv angle, Wair_(act), which is the actual airmass flow rate at the compressor inlet, Wair_(ref), which is thereference air mass flow rate at the compressor inlet, Wexhaust_(act),which is the actual exhaust gas mass flow rate, Wfuel_(act), which isthe fuel mass flow rate, IBH_(fraction), which is the fraction of airbled from the compressor discharge, fa_(ratio ref), which is thereference fuel air mass ratio, LHV_(ref), which is the reference gasfuel's LHV, LHV_(act), which is the actual gas fuel's LHV, sh, which isthe air specific humidity, ha, which is humid air, wv, which is watervapor, da, which is dry air, SG_(xx), which is the specific gravity ofha, wv, or da, ρ_(xx), which is the density of ha, wv, or da, m_(xx),which is the mass of ha, wv, or da, and V_(xx), which is a volume of ha,wv, or da.

The processor may be configured to divide the compressor pressuredischarge by the turbine exhaust pressure to obtain the turbine pressureratio or indentify plural operating points for the gas turbine in theplane defined by the exhaust temperature and the turbine pressure ratio;or apply multiple bilinear interpolations to the identified pluralpoints; and determine a first set of points for a lean gas and a secondset of points for a rich gas. Further, the processor may be configuredto apply a linear interpolation to points in the first set and thesecond set, the points having a same turbine pressure ratio or apply apolytropic correction to a result of the linear interpolation tocalculate a set point exhaust temperature.

According to an exemplary embodiment, a computer readable mediumincluding computer executable instructions, wherein the instructions,when executed, implements a method for controlling an operating point ofa gas turbine that includes a compressor, a combustor and at least aturbine.

According to an exemplary embodiment, the exhaust temperature referencecurve ttxh, the exhaust temperature threshold curve ttxth and othercurves represented in plane (ttx, tpr) may be calculated based on otherparameters that characterize a fuel instead of the lower heating value(LHV). Such parameters may be, for example, a NOx (oxides of Nitrogen)factor, an upper to lower flammability ratio (a lower flammability limitis the smallest percentage of combustible in a given volume of a mixtureof fuel and air (or other oxidant) that supports a self propagatingflame and an upper flammability limit is the highest percentage of thecombustible in the given volume that supports a self propagating flame),etc. In other words, ttxh curve has been calculated in an exemplaryembodiment discussed above as being ttxh=ttxha+Δttxh, wherettxha=ttxhr·(LHV−LHVl)/(LHVr−LHVl)+ttxhl·(LHVr−LHV)/(LHVr−LHVl).However, the ttxha depends on the lower heating value LHV of the fueland not from, for example, the NOx factor, the upper to lowerflammability ratio, etc.

Thus, if a gas turbine is fed sequentially with first and second fuels,which have the same MWI index but different NOx factors, the algorithmdiscussed above for calculating the ttxh is not sensitive to the NOxfactor as this factor is not part of the ttxha function. As the MWIfactor depends from the LHV, which is reflected in the formula for thettxha, ttxha and implicitly the ttxh curve are influenced by a change inthe MWI index of the fuel. However, as the first and second fuels havesimilar MWI indexes, the ttxh curve and other curves based on the LHVvariable will not be able to “see” that different fuels are provided tothe gas turbine.

For this reason, according to an exemplary embodiment, the ttxh, ttxth,and other curves may be calculated as a function of the NOx factor, theupper to lower flammability ratio, or other parameters characteristicsfor a fuel. In one application, the same mathematical functions andalgorithm may be used to calculate the new ttxh, ttxth curves but withthe LHV parameter replaced by the new parameter. However, otherfunctions and/or algorithms may be used to calculate the ttxh, ttxth andother curves based on the NOx factor, the upper to lower flammabilityratio, etc. In other words, the controller 70 may be configured tocalculate the desired curves in multiple (ttx, tpr) planes, eachcorresponding to a given fuel parameter.

For purposes of illustration and not of limitation, an example of arepresentative controller 900 capable of carrying out operations inaccordance with the exemplary embodiments is illustrated in FIG. 9. Thecontroller 70 discussed above with regard to FIG. 2 may have thestructure of controller 900. It should be recognized, however, that theprinciples of the present exemplary embodiments are equally applicableto a processor, computer system, etc.

The exemplary controller 900 may include a processing/control unit 902,such as a microprocessor, reduced instruction set computer (RISC), orother central processing module. The processing unit 902 need not be asingle device, and may include one or more processors. For example, theprocessing unit 902 may include a master processor and associated slaveprocessors coupled to communicate with the master processor.

The processing unit 902 may control the basic functions of the system asdictated by programs available in the storage/memory 904. Thus, theprocessing unit 902 may execute the functions described in FIG. 8. Moreparticularly, the storage/memory 904 may include an operating system andprogram modules for carrying out functions and applications on thecontroller. For example, the program storage may include one or more ofread-only memory (ROM), flash ROM, programmable and/or erasable ROM,random access memory (RAM), subscriber interface module (SIM), wirelessinterface module (WIM), smart card, or other removable memory device,etc. The program modules and associated features may also be transmittedto the controller 900 via data signals, such as being downloadedelectronically via a network, such as the Internet.

One of the programs that may be stored in the storage/memory 904 is aspecific program 906. As previously described, the specific program 906may store relevant parameters of the gas turbine and also may includeinstructions for calculating the ttx_(set point) and sendinginstructions to close or open IGV, etc. The program 906 and associatedfeatures may be implemented in software and/or firmware operable by wayof the processor 902. The program storage/memory 904 may also be used tostore data 908, such as the relevant parameters of the gas turbine, orother data associated with the present exemplary embodiments. In oneexemplary embodiment, the programs 906 and data 908 are stored innon-volatile electrically-erasable, programmable ROM (EEPROM), flashROM, etc. so that the information is not lost upon power down of theparallel computing system 900.

The processor 902 may also be coupled to user interface 910 elementsassociated with a control station in a power plant. The user interface910 of the power plant may include, for example, a display 912 such as aliquid crystal display, a keypad 914, speaker 916, and a microphone 918.These and other user interface components are coupled to the processor902 as is known in the art. The keypad 914 may include alpha-numerickeys for performing a variety of functions, including dialing numbersand executing operations assigned to one or more keys. Alternatively,other user interface mechanisms may be employed, such as voice commands,switches, touch pad/screen, graphical user interface using a pointingdevice, trackball, joystick, or any other user interface mechanism.

The controller 900 may also include a digital signal processor (DSP)920. The DSP 920 may perform a variety of functions, includinganalog-to-digital (A/D) conversion, digital-to-analog (D/A) conversion,speech coding/decoding, encryption/decryption, error detection andcorrection, bit stream translation, filtering, etc. The transceiver 922,generally coupled to an antenna 924, may transmit and receive the radiosignals associated with a wireless device.

The controller 900 of FIG. 9 is provided as a representative example ofa computing environment in which the principles of the present exemplaryembodiments may be applied. From the description provided herein, thoseskilled in the art will appreciate that the present invention is equallyapplicable in a variety of other currently known and future mobile andfixed computing environments. For example, the specific application 906and associated features, and data 908, may be stored in a variety ofmanners, may be operable on a variety of processing devices, and may beoperable in mobile devices having additional, fewer, or differentsupporting circuitry and user interface mechanisms. It is noted that theprinciples of the present exemplary embodiments are equally applicableto non-mobile terminals, i.e., landline computing systems.

The disclosed exemplary embodiments provide a gas turbine and a methodfor controlling the gas turbine based on a new paradigm, e.g., exhausttemperature versus turbine pressure ratio. It should be understood thatthis description is not intended to limit the invention. On thecontrary, the exemplary embodiments are intended to cover alternatives,modifications and equivalents, which are included in the spirit andscope of the invention as defined by the appended claims. Further, inthe detailed description of the exemplary embodiments, numerous specificdetails are set forth in order to provide a comprehensive understandingof the claimed invention. However, one skilled in the art wouldunderstand that various embodiments may be practiced without suchspecific details.

Although the features and elements of the present exemplary embodimentsare described in the embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the embodiments or in various combinations with or withoutother features and elements disclosed herein.

As also will be appreciated by one skilled in the art, the exemplaryembodiments may be embodied in a wireless communication device, acontrol station in a power plant, as a method or in a computer programproduct. Accordingly, the exemplary embodiments may take the form of anentirely hardware embodiment or an embodiment combining hardware andsoftware aspects. Further, the exemplary embodiments may take the formof a computer program product stored on a computer-readable storagemedium having computer-readable instructions embodied in the medium. Anysuitable computer readable medium may be utilized including hard disks,CD-ROMs, digital versatile disc (DVD), optical storage devices, ormagnetic storage devices such a floppy disk or magnetic tape. Othernon-limiting examples of computer readable media include flash-typememories or other known memories.

Although the features and elements of the present exemplary embodimentsare described in the embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the embodiments or in various combinations with or withoutother features and elements disclosed herein. The methods or flow chartsprovided in the present application may be implemented in a computerprogram, software, or firmware tangibly embodied in a computer-readablestorage medium for execution by a specifically programmed computer orprocessor.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other example are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements within the literal languages ofthe claims.

1. A method for controlling an operating point of a gas turbine thatincludes a compressor, a combustor and at least a turbine, the methodcomprising: determining a turbine exhaust pressure at an exhaust of theturbine; measuring a compressor pressure discharge at the compressor;determining a turbine pressure ratio based on the turbine exhaustpressure and the compressor pressure discharge; calculating an exhausttemperature at the exhaust of the turbine as a function of the turbinepressure ratio; identifying a reference exhaust temperature curve in aplane defined by the exhaust temperature and the turbine pressure ratio,wherein the reference exhaust temperature curve includes those pointsthat are optimal for operating the gas turbine; and controlling the gasturbine to maintain the operating point on the reference exhausttemperature curve.
 2. The method of claim 1, further comprising:adjusting an angle of inlet guide vanes disposed at an inlet of thecompressor to maintain the operating point on the reference exhausttemperature curve.
 3. The method of claim 1, further comprising: usingthe reference exhaust temperature curve to control a firing temperatureof the combustor, wherein the firing temperature is the temperature ofthe combustion products downstream a first stage nozzle of the turbine.4. The method of claim 1, wherein determining a turbine exhaust pressurecomprises: calculating an exhaust pressure drop due to a mass flowing inthe flue of the turbine; calculating a pressure recovery due to achimney effect, wherein the chimney effect is due to an elevationdifference between the exhaust of the turbine and a flue discharge toatmosphere; and adding together the exhaust pressure drop due to themass flowing and the pressure recovery to obtain the turbine exhaustpressure.
 5. The method of claim 1, wherein the turbine exhaust pressuredepends from ρ_(exhaust), which is a density of the exhaust gas at anactual exhaust temperature and actual ambient pressure, ρ_(exhaust ref),which is a density of the exhaust gas at the reference exhausttemperature and a reference ambient pressure, ρ_(air), which is adensity of the ambient air at the actual pressure and temperature,ρ_(air ref), which is a density of the ambient air at the referencepressure and temperature, Δh, which is the elevation difference betweenthe gas turbine exhaust and the flue discharge to the atmosphere, v,which is the exhaust speed inside the flue, ttx_(ref), which is thereference exhaust temperature, ttx_(act), which is the actual exhausttemperature, pamb_(ref), which is the reference ambient pressure,pamb_(act), which is the actual ambient pressure, pinlet_(act), which isthe actual air pressure at the compressor inlet, pinlet_(ref), which isthe reference air pressure at the compressor inlet, tamb, which is theambient temperature, tinlet_(act), which is the actual air temperatureat the compressor inlet, tinlet_(ref), which is the reference airtemperature at the compressor inlet, tnh_(act), which is the compressoractual speed, tnh_(ref), which is the compressor reference speed,igv_(act), which is the actual igv angle, igv_(ref), which is thereference igv angle, Wair_(act), which is the actual air mass flow rateat the compressor inlet, Wair_(ref), which is the reference air massflow rate at the compressor inlet, Wexhaust_(act), which is the actualexhaust gas mass flow rate, Wfuel_(act), which is the fuel mass flowrate, IBH_(fraction), which is the fraction of air bled from thecompressor discharge, fa_(ratio ref), which is the reference fuel airmass ratio, LHV_(ref), which is the reference gas fuel's LHV, LHV_(act),which is the actual gas fuel's LHV, sh, which is the air specifichumidity, ha, which is humid air, wv, which is water vapor, da, which isdry air, SG_(xx), which is the specific gravity of ha, wv, or da,ρ_(xx), which is the density of ha, wv, or da, m_(xx), which is the massof ha, wv, or da, and V_(xx), which is a volume of ha, wv, or da.
 6. Themethod of claim 1, wherein determining the turbine pressure ratiocomprises dividing the compressor pressure discharge by the turbineexhaust pressure to obtain the turbine pressure ratio.
 7. The method ofclaim 1, wherein calculating the exhaust temperature comprises:identifying plural operating points for the gas turbine in the planedefined by the exhaust temperature and the turbine pressure ratio;applying multiple bilinear interpolations to the identified pluralpoints; and determining a first set of points for a lean gas and asecond set of points for a rich gas.
 8. The method of claim 1, furthercomprising: applying a linear interpolation to points in the first setand the second set, the points having a same turbine pressure ratio. 9.The method of claim 8, further comprising: applying a polytropiccorrection to a result of the linear interpolation to calculate a setpoint exhaust temperature.
 10. A gas turbine having a control device forcontrolling an operating point of the gas turbine, the gas turbinecomprising: a compressor configured to compress a fluid; a combustorconnected to a discharge of the compressor and configured to mix thecompressed fluid with fuel; at least a turbine connected to thecompressor and configured to expand burnt gas from the combustor togenerate power to an output of the gas turbine; a pressure sensorprovided at the discharge of the compressor to measure a compressorpressure discharge; and a processor that communicates with the pressuresensor and is configured to, determine a turbine exhaust pressure dropat an exhaust of the turbine, determine a turbine pressure ratio basedon the turbine exhaust pressure and the compressor pressure discharge,calculate an exhaust temperature at the exhaust of the turbine as afunction of the turbine pressure ratio, identify a reference exhausttemperature curve in a plane defined by the exhaust temperature and theturbine pressure ratio, wherein the reference exhaust temperature curveincludes those points that are optimal for operating the gas turbine,and control the gas turbine to maintain the operating point on thereference exhaust temperature curve.